The present application relates to a storage element including a storage layer, in which a magnetization state of a ferromagnetic layer is stored as information, and a fixed magnetization layer, a magnetization direction of which is fixed, where the magnetization direction of the storage layer can be changed by applying a current in the direction perpendicular to the plane of the film to inject spin-polarized electrons. The present application also relates to a memory including such storage element, and can be favorably applied to a nonvolatile memory.
High-speed, high-density DRAM is widely used as random access memory in computers and other information devices.
However, because DRAM is volatile memory, the information within which disappears when power is turned off. Hence, there is a demand for nonvolatile memory in which information may not disappear when there is no power.
As such a nonvolatile memory, magnetic random access memory (MRAM), in which information is recorded using magnetization in a magnetic material, is attracting attention and is currently under development.
In MRAM, currents flow through two types of substantially perpendicular address lines (word lines and bit lines), and information is recorded by inverting magnetization in a magnetic layer of a magnetic storage element at an intersection of the address lines using an electric current-induced magnetic field generated by the address lines. When information is read, magnetoresistive effect (MR effect) is used, in which the resistance changes according to the direction of magnetization in the storage layer of the magnetic storage element.
FIG. 1 shows a schematic (perspective) view of a typical MRAM device.
Drain regions 108, source regions 107, and gate electrodes 101, forming selection transistors to select memory cells, are formed in portions separated by element separation layers 102 in a silicon substrate or other semiconductor substrate 110. Above the gate electrodes 101 are provided word lines 105 extending in the front-back direction in the figure.
The drain regions 108 are formed so as to be shared by selection transistors on the left and right in the figure. Lines 109 are connected to the drain regions 108.
Between the word lines 105 and the bit lines 106, which are positioned above the word lines 105 and extend in the left-right direction in the figure, are positioned magnetic storage elements 103 having a magnetic layer the magnetization direction of which is inverted. These magnetic storage elements 103 include, for example, magnetic tunnel junction (MTJ) elements. Further, the magnetic storage elements 103 are electrically connected to the source regions 107 through horizontal-direction bypass lines 111 and a contact layer 104 in the vertical direction. By passing currents through a word line 105 and a bit line 106, a current-induced magnetic field is applied to a magnetic storage element 103, thereby inverting the direction of magnetization in the storage layer of the magnetic storage element 103 and information can be recorded.
In MRAM and other magnetic memories, in order to store recorded information stably, the magnetic layer (storage layer) in which information is recorded may need to have a constant coercive force. On the other hand, in order to overwrite recorded information, a certain amount of electric current may need to pass through address lines. However, since elements forming MRAM are made finer, address lines also grow narrower, so that it is difficult to pass a sufficiently large current.
Hence, in order to invert magnetization using small currents, memory configured to use magnetization inversion caused by spin injection has been attracting attention (see, for example, Japanese Unexamined Patent Application Publication No. 2003-17782). Magnetization inversion caused by spin injection involves injecting spin-polarized electrons passed through a magnetic material into another magnetic material, resulting in torque generated on the other magnetic material and causing the magnetization inversion.
For example, by passing current in the direction perpendicular to the plane of a film of a giant magnetoresistive effect (GMR) element or a magnetic tunnel junction (MTJ) element, the direction of magnetization in at least part of the magnetic layers of these elements can be inverted.
Moreover, magnetization inversion by spin injection has such an advantage that the magnetization inversion is effected without increasing the current, even if the element is very small.
FIGS. 2 and 3 are schematic views of memory configured to use the magnetization inversion caused by spin injection as described above. FIG. 2 is a perspective view, and FIG. 3 is a cross-sectional view.
Drain regions 58, source regions 57, and gate electrodes 51, forming selection transistors used to select memory cells, are each formed in portions separated by an element separation layer 52 of a silicon substrate or other semiconductor substrate 60. The gate electrodes 51 also serve as word lines extending in the front-back direction in FIG. 2.
The drain regions 58 are formed so as to be shared by selection transistors on the right and left in FIG. 2. Lines 59 are connected to these drain regions 58.
Further, storage elements 53, having a storage layer the magnetization direction of which is inverted by spin injection, are positioned between the source regions 57, and the bit lines 56 positioned above the source regions 57 and extending in the right-left direction in FIG. 2.
Such a storage element 53 includes, for example, a magnetic tunnel junction (MTJ) element. As shown in the figure, magnetic layers 61 and 62 are provided. One of the magnetic layers 61 and 62 is a fixed magnetization layer the magnetization direction of which is fixed, and the other magnetic layer is a free magnetization layer, that is, a storage layer, the magnetization direction of which changes.
The storage element 53 is connected to a bit line 56 and a source region 57 through a vertical contact layer 54. Accordingly, current is passed through the storage element 53 to cause inversion of the direction of magnetization in the storage layer by spin injection.
Memory configured to use magnetization inversion caused by spin injection has the feature of enabling the device structure to be simplified compared with typical MRAM shown in FIG. 1.
Further, when using magnetization inversion caused by spin injection, there is the advantage that the write current is not increased, even when the element size is reduced, compared with typical MRAM in which magnetization inversion is effected by an external magnetic field.
In the case of MRAM, write lines (word lines and bit lines) are provided separately from the storage elements, and by passing a current through the write lines to generate a current-induced magnetic field, information is written (recorded). Accordingly, a sufficiently large current for writing may be passed through the write lines.
On the other hand, in memory configured to use magnetization inversion caused by spin injection, spin injection may need to be carried out by passing a current through storage elements to invert the direction of magnetization in the storage layer.
Because information is written (recorded) by directly passing a current through storage elements as described above, storage elements are connected to selection transistors so that memory cells for writing can be selected. Here, the current flowing through a storage element is limited to the current which can be passed through a selection transistor (saturation current of the selection transistor).
Accordingly, information may need to be written using a current equal to or less than the saturation current of selection transistors. Therefore, the efficiency of spin injection may need to be improved so that current passed through storage elements may be reduced.
Further, in order to obtain a large read signal, a large magnetoresistive change rate may need to be secured. Hence, it is effective to provide a storage element in which a tunnel insulating layer (tunnel barrier layer) serves as an intermediate layer in contact with both sides of the storage layer.
When using a tunnel insulating layer as an intermediate layer as described above, an amount of current passed through the storage element may need to be limited to prevent dielectric breakdown of the tunnel insulating layer. In light of this also, the current during spin injection may need to be controlled.
Hence, storage elements configured to invert the magnetization direction of the storage layer by spin injection may need to improve the spin injection efficiency and reduce the current required.